Offshore seismic systems today comprise relatively complex combinations of on-board systems and towed arrays. A towed array includes signal sources and receivers, as well as radio-telemetry systems, navigation systems, and other devices. For proper operation, the towed array must be closely maintained within a defined depth range, and the maintenance of this depth is most commonly accomplished with depth control devices attached to or coupled in-line with the array streamers. Such a depth control device is often referred to as a "bird".
A depth control device includes electronics which require a specific voltage of electrical power, generally 3 V.sub.DC, as well as a motor for driving depth control planes or wings, which requires a different voltage, generally 6 V.sub.DC, for proper operation. In current seismic streamer systems, these voltages are provided by a battery pack with 3 and 6 volt battery cells. With use, the battery packs are drained and require replacement with fresh battery packs. To accomplish this, the streamer is reeled in, the depth control device is removed from the streamer and opened up to remove the expended battery pack and to place a fresh battery pack in its place.
Unfortunately, opening up the bird exposes its internal electronics to the deleterious effects of salt-laden air. Also, even if the battery pack is replaced in a carefully controlled environment, after the fresh battery pack is installed the access cover to the battery pack compartment may not be properly sealed and may consequently leak when the bird is re-deployed.
Furthermore, even if the interior of the bird is not exposed to salty air and does not leak when re-submerged, the process of removing, recharging, and replacing a battery pack is labor intensive and time consuming. Any such activities on board seismic vessels increases the time and cost of the seismic exploration and is therefore to be avoided or reduced, if possible.
Thus, there remains a need for a rechargeable or replaceable battery pack for a depth control device. Such a battery pack may be rechargeable with the entire bird assembled where the recharging operation takes place when the bird is stored in a storage rack between seismic exploration operations. Alternatively, the battery pack may be detached from the main body of the bird, without the need for access to the interior of the bird. The battery pack may then be placed in a storage rack and recharged while the battery pack is in storage. While the detached battery pack is being charged in the storage rack, replacement battery packs are installed in the birds and the seismic streamers are deployed for the continuation of seismic exploration operations without delay.